# Pressure Limit Versus Flow / Composition

<p><span>Hi, </span></p><p></p><p><span>I recently set up a fast method with a 30 mm column that i have scaled from a 50 mm column. But get this notice.... </span></p><p></p><p><span>"The flow and composition profile of this gradient requires that the system pressure be limited to 11538 psi", "the high pressure limit will be adjusted".</span></p><p></p><p><span>Why is this, are there problems with measuring pressure when composition is changing rapidly at higher flow rates?. Gradient program below:</span></p><p></p><p><span><u><table border="1"><tbody><tr><td><strong><u>Time </u></strong></td><td><strong><u> Flow (ml/min)</u></strong></td><td><strong><u> % ACN</u></strong></td><td><strong><u> Curve</u></strong></td></tr><tr><td> 0</td><td> 1.66</td><td> 5</td><td> Initial</td></tr><tr><td> 1.00</td><td> 1.66</td><td> 50</td><td> 6</td></tr><tr><td> 1.81</td><td> 1.66</td><td> 5</td><td> 1</td></tr></tbody></table></u></span></p><p><span>The method seems to work okay regardless.</span></p><p></p><p><span>Kind regards,</span></p><p></p><p><span>Jason</span></p>

• ithink this message due to sharp change in delta psi (pressure could be from 13500 to 11000 psi at 40C) as your psi may be around 13000-13500 psi at 5% & 11000psi at 50%.... it will disturb compression/decompression ratio rapidly.... did you try curve 9 instead of 1........ ? as after 1.00m from 50-5% approaches rapidly...

& i think your time is little long for 30mm column.... it should be around 1.0-1.1m at 1.66 ml/m flow

~~ Salman

• Dont think i quite get that explanation....are you saying that the pressure would rise too quickly for the pressure limt to prevent damage?. Thats another theory i have anyway.

Also wouldnt curve 9 create an even more ballistic gradient as it nears 1 minute.

Why is my time too long for the 30 mm column?, surely this is analyte dependant, if it was shorter some compounds would appear in the next injection.

• I never worked at 1.66 flow, I used max 1.2 flow with 30mm column get four peaks in 1 min. that’s why im slightly watchful about time, 1 min is enough for 4-5 peaks at this high flow, actually 5-10% ACN for equilibration at start & at end, as you said after 1.0m at curve 1 final composition change will be ballistic & at this pressure you must be careful (approaching 95% aqueous). If you like, you can try 0m=5%, 0.8m=50% & 1.2m=5% (curve 9 or 7) and run time could be 1.4m with same flow (even i dont know what is your analytes)

I don’t know why this message appears, but may be this is for caution… as one of my colleague said for 30mm column above 10000psi is not in practical, better answer will give you waters co-workers.

~~ Salman

• Hello:

In terms of the instrument control this does seem about right.

At flow rates above 1 ml/min, the allowed pressure limit is linearly reduced to 9,000 psi at 2.00 ml/min. 12,000 psi sound about right for a flow of 1.66 ml/min.

Liz

• Thanks Liz,

it seems to be related to composition also, i.e. i get different pressure limits for different gradients using the same flow rate .

I have to admit i am still curious as to the reason behind this , is it pump issues or pressure Xducer limitations.

• Its not an "issue" - its physics

Absolutely, composition will affect pressure, viscosity if very different for different solvents and temperatures. So for a gradient where the composition is slightly different due to the gradient of the composition curve will show a pressure difference.

That's why the pump has a really smart alogorithm because as you mix you get a reaction, which can generate considerable exothermic/frictional heat at the pressures in UPLC, (and viscosity differs with pressure, a pump calibrates to a different flow with sea level variations - another good reason to visit Peru) temperature changes and viscocity changes and thus so does volume delivered. The pump is constantly correcting for this, changing the actual volume of the stroke.

This gets even more complicated as the chromatography laws come into effect - if you run at higher temperatures the flow rate to attain van deemter optimum is higher. So if you run at 1 mL/min at 80 degrees C, to attain the same peak capacity you need only run at 0.84 mL/min at 80 degrees. If the separation was in methanol, you might need to run at 0.87 mL/min for ACN.

You are simply pushing the pump right at the edge of pressure rating, so if you back off on the flow rate a little the messgae will go away.

Liz

• So it is a pump issue lowering pressure limits at higher flow rates, with the specific issue being heating within the pump?. As the heating is essentially diabatic, this causes an issue with higher pressure inside the pump (which is not measured) than for the rest of the system?.... sorry i am having trouble following the explanation.

The explanation doesnt really matter (nice to have tho), just want to be sure that the column and other system components are okay for longer term use at the flow rate given & what the limiting factor actually is. From what i can gather, the pump may be experiencing higher pressures than observed, but the pressure on the rest of the system is as measured, and columns are going to be exposed to normall presures.

For routine use, i have scaled the flow rate back to 1.5 ml/min. This gives a limit of 12450 psi.

Cheers,

Jason

• Hello,

Each pump is specified as rated to 2 mL up to 9,000 psi and 1mL/min to 15,000 psi. At 1.66mL/min one of the pump heads is being asked to operate to its limits. It is simply that you have are close to exceeding the backpressure ratings for the binary solvent manager and therefore are attempting to run outside the recommedations and thus outside where the unit has been tested. The BSM firmware is setup so the pump does not allow a user to exceed the specifications, so you cannot exceed the limits, but you do get the message that informs you that you are at the limit. If the pump allowed you to run above the specifications you could indeed run outside reliable perfromance parameters for the pump (and pumping algorithm), so this is disabled. Scaling back to 1.5 mL/min

is getting you back within the pressure envelope for your conditions.

My apolgies for being unclear and I hope that this is a better explanation!

Liz